Wednesday, May 22, 2013

Why warming reduces the occurrence and intensity of tornadoes

In a post today by Andy Revkin, Dr. Harold Brooks of the National Severe Storms Laboratory points out that "There are more F1+ tornadoes in warm winter months and fewer in warm summer months." and that "there are ~20% fewer tornadoes in the collection of warm months." Dr. Brooks also notes, "Probability of occurrence is mostly driven by wind shear and intensity is almost completely independent of the thermodynamics. The observations are clear on that. As a result, expected changes in occurrence and intensity would be driven by wind shear changes" and that 2/3 of the models indicate "a decrease in [wind] shear over the US."

1. Interannual variability is incredibly large. It will be very difficult to detect long-trend trends. In the last 3 years, we’ve set records for the most F1+ tornadoes (back to 1954) in a 12 consecutive month period and for the fewest F1+ tornadoes. I think there’s evidence to suggest that we have seen an increase in the variability of occurrence in the US.

2. Probability of occurrence is mostly driven by wind shear and intensity is almost completely independent of the thermodynamics. The observations are clear on that. As a result, expected changes in occurrence and intensity would be driven by wind shear changes. NOAA is doing some new work on this, but Brian Soden indicated to me that ~2/3 of the CMIP runs showed an increase in CAPE and a decrease in shear over the US.

3. There are more F1+ tornadoes in warm winter months and fewer in warm summer months. Given that there are more in the mean in the summer than winter, overall, if we take the warmest 10 Januarys, 10 Februarys, etc. back to 1954 and count the tornadoes in them and compare it to the coolest 10 Januarys, 10 Februarys, etc., there are ~20% fewer tornadoes in the collection of warm months.